1. Field of the Invention
The present invention relates to a photo mask with an electric-discharge (ESD) protective function, and more particularly, to a photo mask that discharges static electricity into the air through discharging peaks.
2. Background of the Invention
Photolithography is an important step in the process of semiconductor production, and many things associated with MOS transistors, such as film patterns and dopant regions, are determined by this step Photolithography proceeds basically by covering a layer of photosensitive material, and casting a parallel light beam through a glass mask onto the photosensitive material. Because of the pattern of the circuit layout on the photo mask, the photosensitive material undergoes a selective photo reaction so as to transfer the pattern on the photo mask onto the semiconductor wafer.
In general, the photo mask comprises a smooth and transparent plate of glass or quartz as its foundation and a layer of chromium film 1000 xc3x85 thick over the surface of the photo mask. The pattern with a transparent-opaque layout on the photo mask is etched onto the chromium film for pattern-transferring. But in the process of using or storing the photo mask, static electricity may be induced, forming an electric field on the photomask. The electric field attracts particles in the air to the photo mask or undergoes a neutralizing discharge reaction on the surface of the photo mask, resulting in burn marks. Consequently, the pattern transferred through the photo mask can lose its clarity.
Please refer to FIG. 1, which is a prior art photo mask 10. The prior art photo mask with an ESD protective function comprises a transparent quartz substrate 12, a pattern area 14 positioned on a predetermined area on the surface of the quartz substrate 12 for pattern-transferring, an inner chromium film 16 positioned on the surface of the quartz substrate and around the pattern area 12, an ESD annulus 18 comprising an exposed quartz substrate positioned on the surface of the transparent quartz substrate 12 and around the inner chromium film 16, and an outer chromium film 20 on the surface of the quartz substrate 12 and the ESD annulus 18. The ESD annulus 18 is an insulated layer for insulating the inner chromium film 16 and the outer chromium fill 20.
When the photo mask 10 touches a user, electrostatic charges are induced on the outer chromium film 20. The electrostatic charges on the outer chromium film 20 induce the opposite inductive charges on the inner chromium film 16 Since the inner chromium film 16 and the pattern area 14 are electrically neutral, the pattern area 14 also carries the equal amount of the opposite charge upon the induction of the inductive charges on the inner chromium film 16.
Consequently, an electric field is formed in the pattern area 14 that draws particles in the air onto the pattern area 14 or has a neutralizing discharge reaction with the air particles, leaving burn marks in the pattern area 14. Also, the neutralizing discharge reaction results in high heat in the thin end of the circuit pattern, damaging the pattern. Once inductive charges in the outer chromium film 20 accumulate to a certain level, they have a neutralizing discharge reaction with the opposite charges of the inner chromium film 16 as well, canceling out the accumulated charges and restoring the inner and outer chromium films 16 and 20 to neutral. But in the neutralizing discharge reaction, the charges unload their electrical energy to reach a more stable state. The electrical energy turns into heat, making burn marks in the thin circuit end of the pattern area 14, damaging the pattern.
Present semiconductor manufacturing processing can be as small as 0.15 xcexcm, and the damage to the circuit pattern caused by the neutralizing discharge reaction will only get worse as process sizes shrink. Every time damage to the circuit pattern occurs, the photo mask, and sometimes even the entire semiconductor wafer, has to be discarded. Consequently, such damage affects the quality of pattern transfer, severely lowers productivity, and shortens the life of the photo mask.
The object of the claimed invention is to solve the problem described above by providing a photo mask with an electric discharge protective function, wherein static electricity is discharged into the air through discharging peaks by means of a neutralization discharge reaction so as to prevent the thin circuit end of the circuit pattern on the photo mask from being damaged by the neutralizing discharge reaction.
The claimed invention provides a photo mask with an ESD protective function that comprises a transparent substrate comprised of quartz or glass with a thickness of 1000xcx9c1200 angstroms, a patterned chromium shielding layer on the transparent substrate, and an ESD protective thin layer of chromium film 1000xcx9c1200 angstroms thick positioned on the surface of the transparent substrate and around the shielding chromium layer. The ESD protective layer has a plurality of discharging peaks with the function of discharging the static electricity in the ESD protective layer into the air, resulting in a neutralizing discharge reaction.
Moreover the presented area in the photomask of the claimed invention has a chromium film of 1000xcx9c1200 xc3x85 thick, whose outer fringe comprises peaks arrayed in a saw-toothed structure, pointing outward, which can discharge the static electricity in the ESD protective layer into the air so as to remove charges from the photo mask.
Because of the outward-pointing peaks in the outer area of the photo mask, the static electricity induced in the photo mask concentrates and forms electrical fields on the peaks instead of lingering in the patterned area. When the static electricity accumulates to a certain level in the peaks, it is discharged into the air and neutralized. Furthermore, the high heat that results from the neutralizing discharge reaction will only affect the protected area and will not damage the circuit pattern in the pattern area.